Electronic components such as integrated circuits, multi-chip modules, passive components and radio frequency (RF) power transistors may be heat sources which require cooling during normal operation.
Traditionally, electronic components have been cooled by conduction and natural or forced air convection. Advances in electronics, however, have resulted in some devices, such as radio frequency power devices, having power densities which exceed the capabilities of traditional natural or forced convective air cooling.
Two-phase cooling is an example of a thermal management technique which uses liquid coolants, rather than conductive, solid heat spreaders and air, to dissipate heat generated by electronic components. Evaporative spray cooling is a type of two-phase cooling which features the spraying of atomized droplets of a dielectric fluid such as a perfluorocarbon fluid directly or indirectly onto a surface of an electronic component such as a RF power transistor. When the fluid droplets impinge upon the transistor's surface, a thin film of liquid coats the transistor, and heat is removed primarily by evaporation and/or evaporation of the perfluorocarbon fluid from the transistor's surface.
Evaporative spray cooling is a preferred method of heat removal in many electronics applications, and perfluorocarbon dielectric fluids are generally chemically inert and stable. In the case of a malfunction of the spray cooling system or of the electronic component which is being cooled, however, temperatures of 250 degrees Celsius may be exceeded for a period of time, and toxic thermal decomposition products such as perfluoroisobutylene (PFIB) and hydrogen fluoride (HF) may be generated. And when components such as RF power transistors are being cooled, the risk of generating excessive temperatures may be increased due to high heat flux of these devices.
One well-known temperature-sensitive failure management system includes a logic circuit such as a microprocessor which communicates with a temperature sensor such as a thermocouple or thermistor. In operation, the thermocouple generates a signal corresponding to a temperature at a particular location and transmits the signal to the microprocessor. If the microprocessor determines that a predetermined temperature threshold has been exceeded, it may activate an alarm or interrupt a power supply.
Logic-based systems suffer from various problems. For example, such systems are dependent on the performance of the very electronics which they are intended to monitor--excessive temperatures could disable the system itself if thermal sensors located too close to the source of excessive heat are damaged or if the microprocessor is disabled due to high temperatures. Locating thermal sensors a safe distance away from the heat source, however, may delay notification that the predetermined temperature threshold has been reached. In addition, thermocouples and other sensors are electrical in nature and may therefore be incompatible with RF and other high-power devices because of electromagnetic interference effects.
Another common method for managing fluid cooling systems involves the use of special fluid filters which, in a closed system, convert a potentially dangerously decomposed fluid into a safe form. Special fluid filters are expensive, however, and may become saturated, and possibly unreliable, over time. Although chemical sensors which monitor for the presence of dangerous decomposition products such as PFIB and HF may also be utilized, such sensors may not be sensitive enough to protect human beings from the risk of exposure to dangerous substances.
Discrete devices, such as positive temperature coefficient (PTC) resistors, which significantly increase their resistance when an overcurrent condition develops, may not be compatible with RF devices and thus may have to be distanced from a perfluorocarbon fluid-cooled RF heat source. As such, PTC resistors may not react quickly enough to avert the potentially dangerous consequences of increases in temperature.
There is therefore a need for an electronic device and for a method for regulating an amount of power delivered to a radio frequency electronic component which reduces the safety risks associated with cooling the component using a fluid such as a perfluorocarbon fluid.